Herbs, annual, cespitose. Culms trigonous, 2-30 cm × 0.6-1.1 mm, glabrous. Leaves flat, 4-10 cm × 2-4 mm. Inflorescences: rays 1-3, 0.2-1.5 cm; 2d order rays present in robust plants, to 5 mm; bracts 2-3, longest erect, others spreading, 1-20 cm × 1.5-3 mm. Spikelets 3-12, narrowly ellipsoid, flattened, 3-7 × 0.9-1.2 mm; floral scales 8-12(-16), laterally purplish brown, medially greenish yellow, 3-ribbed medially, orbiculate, 0.9-1.1 × 1 mm, apex mucronate. Flowers: stamen 1; anthers ellipsoid, 0.2 mm, connective not prolonged; styles 0.3-0.4 mm; stigmas 0.3 mm. Achenes light brown, ellipsoid, 0.7-0.9 × 0.4 mm, base barely stipelike to narrowly cuneate, apex acute, surfaces glabrous. Fruiting summer. Damp, disturbed soils, emergent shorelines, puddles; 0-1700 m; introduced; Ont., Que.; Calif., Conn., Md., Mass., Mo., Nebr., Nev., N.J., Pa., S.Dak., Va.; Eurasia. A native of temperate Eurasia, Cyperus fuscus is intermittently adventive and locally established 35°-45° N latitude. The report from New York (M. L. Fernald 1950) is based on a misidentification of C. diandrus (R. S. Mitchell and G. C. Tucker 1997).
Tufted annual 1-4 dm; lvs 1-4 mm wide; bracts 2-5, surpassing the infl; rays few, 1-3 cm; spikelets 3-6 mm, 10-20(-40)-fld; rachilla not winged; scales ovate, 0.9-1.5 mm, with reddish-brown sides, keeled, shortly mucronate; stamens 2; achenes whitish, trigonous, 1 mm; 2n=72. Native of Eurasia, intr. at scattered stations in the e. part of our range.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.